[Haskell-cafe] existential types and cast

[Corentin Dupont]

Thanks Paolino, I will try to see how I can use your implementation!
Corentin

On Wed, Jul 4, 2012 at 9:24 PM, Paolino
<paowolo.veronelli at gmail.com<paolo.veronelli at gmail.com>
> wrote:

> Hi Corentin,
> This is how I would model your request (without concrete constructors for
> Player and Rule)
> I'm sure there are better descriptions also  as I'm not an expert.
>
> paolino
>
> {-# LANGUAGE DataKinds, GADTs, KindSignatures #-}
>
> data Player
> data Rule
>
> data Data = Player | Rule
> data EventKind  = Action | Reaction
>
> data Event :: EventKind -> * where
>   NewPlayer  :: Player -> Event Action
>   NewRule    :: Rule -> Event Action
>   NewHandler :: (Event Action -> IO ()) -> Event Reaction
>
> handle ::  Event Action -> Event Reaction -> IO ()
> handle x (NewHandler f) = f x
>
> reaction :: Event a -> [Event Reaction] -> IO [Event Reaction]
>
> reaction f@(NewHandler _) es = return $ f:es
> reaction p@(NewPlayer _) es = mapM_ (handle p) es >> return es
> reaction r@(NewRule _) es = mapM_ (handle r) es >> return es
>
>
> 2012/7/4 Corentin Dupont <corentin.dupont at gmail.com>
>
>> Hi,
>> for example, in my game (Nomic) if a new player arrives, I trigger a
>> "NewPlayer" event. All handlers registered for that event should be
>> triggered, and passed a structure "Player" containing all the infos of the
>> incoming player.
>> If there is a new rule submitted, that the same: the event "NewRule" is
>> triggered and the handlers are passed a structure "Rule". Thus I want the
>> handlers registered on NewPlayer to have the type Player -> xxx, and on
>> NewRule to have the type Rule -> xxx. I want to be able to associate an
>> arbitrary data type (here Player and Rule) to an event.
>> The handlers are inherently of different types, but I want to store them
>> in a unique list hence the existential...
>>
>>
>> On Wed, Jul 4, 2012 at 4:33 PM, Paolino <paolo.veronelli at gmail.com>wrote:
>>
>>> Hi Corentin,
>>> If you could explain *why* there should be a type associated to each
>>> event value, it would help, maybe.
>>> If it's a design choice , maybe it's wrong design. One reason to use
>>> dynamic typing would be to plug  in new type of events. But if you already
>>> have the events semantics , this is not useful.
>>> If the language of events is complex , possibly recursive, you can use
>>> GADTs to enforce their validity by construction and you don't need to
>>> typefy the event values, but some of their characteristics.
>>> Remember type machinery is good to give correctness at the compilation
>>> time which Typeable defeats moving checks at runtime. So lifting values to
>>> types and eliminating this information with existentials and casting seems
>>> wrong.
>>>
>>> paolino
>>>
>>> 2012/7/4 Corentin Dupont <corentin.dupont at gmail.com>
>>>
>>>>  Hi Paolino,
>>>> the user can add as many handlers he wants for each event.
>>>> When a event is triggered along with a data, all handlers associated to
>>>> that event should be triggered and passed the data.
>>>> The trick is, there is one type of data associated with each event.
>>>> That's why I cannot use a Event datatype: how to associate a data type to
>>>> each event value? This would be some sort of dependant typing if I'm not
>>>> mistaken.
>>>> That's why my events exists both on type level and value level:
>>>> *data NewPlayer = NewPlayer
>>>> *
>>>> wich allows me to associate it a typf data with type indexing.*..
>>>> *
>>>>
>>>> Regards
>>>> Corentin
>>>>
>>>>
>>>> On Wed, Jul 4, 2012 at 12:58 PM, Paolino <paolo.veronelli at gmail.com>wrote:
>>>>
>>>>> Hi
>>>>> How many handlers for each type of event in the list of handlers ?
>>>>> If you have only one handler for each type , it should go in the
>>>>> typeclass, and you don't need typeable.
>>>>> If you have more than one maybe you can avoid using type indexing at
>>>>> all, because it doesn't resolve the handler selection issue.
>>>>> By the way , it's not clear to me why you don't have a simple Event
>>>>> datatype describing all the possible events in advance.
>>>>>
>>>>> Regards
>>>>>
>>>>> paolino
>>>>>
>>>>> 2012/7/3 Corentin Dupont <corentin.dupont at gmail.com>
>>>>>
>>>>>> Hi all,
>>>>>> I read somewhere (here:
>>>>>> http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existential-type)
>>>>>> that it's bad to try to unbox an existential type using a cast. OK, but
>>>>>> without I really can't figure out how to do what I want:
>>>>>>
>>>>>> *data NewPlayer = NewPlayer deriving (Typeable, Eq)
>>>>>> data NewRule = NewRule deriving (Typeable, Eq)
>>>>>>
>>>>>> class (Eq e, Typeable e) => Event e where
>>>>>>     data EventData e
>>>>>>
>>>>>> instance Event NewPlayer where
>>>>>>     data EventData NewPlayer = P Int
>>>>>>
>>>>>> instance Event NewRule where
>>>>>>     data EventData NewRule = R Int
>>>>>>
>>>>>> instance Typeable1 EventData where
>>>>>>     typeOf1 _ = mkTyConApp (mkTyCon "EventData") []
>>>>>>
>>>>>> data EventHandler = forall e . (Event e) => EH e (EventData e -> IO
>>>>>> ())
>>>>>>
>>>>>> addEvent :: (Event e) => e -> (EventData e -> IO ()) ->
>>>>>> [EventHandler] -> [EventHandler]
>>>>>> addEvent e h ehs = (EH e h):ehs
>>>>>>
>>>>>> triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] ->
>>>>>> IO ()
>>>>>> triggerEvent e d ehs = do
>>>>>>     let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs
>>>>>>     case r of
>>>>>>        Nothing -> return ()
>>>>>>        Just (EH _ h) -> case cast h of
>>>>>>         Just castedH -> castedH d
>>>>>>         Nothing -> return ()*
>>>>>>
>>>>>> How to remove the casts from triggerEvent? All that I want is to
>>>>>> apply the handler found on the data passed in parameter.
>>>>>> I tried to add a function apply in the class, without success:
>>>>>> *apply :: (EventData e -> IO ()) -> (EventData e) -> IO ()
>>>>>> apply = ($)*
>>>>>>
>>>>>>
>>>>>> Thanks!
>>>>>> Corentin
>>>>>>
>>>>>> _______________________________________________
>>>>>> Haskell-Cafe mailing list
>>>>>> Haskell-Cafe at haskell.org
>>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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